james94

does anyone remember which jet BIG22BIRD used?

DRAGONFIRE81M

29MM grupner http://www.rcuniverse.com/forum/m_65...59/key_/tm.htm

headhunter23

Check out the props on the kehrer site, they show all the props for the different sizes. I haven't checked to see the pitch on mine because I don't know how to. Would have to search it up and pull out one o the props.

After doing some testing theres no way that guy is anywhere near 80km/hr, I'm willing to bet it's 28km/h or less which is insane for all the money he dumped in the motors and it's not even as peppy as the jet villain and it does 29km on 14.8v.

Finally got some new numbers. Now the villain is a little larger than the delta jet, 33vs29, weight difference is 1352 vs 1102(no batt), delta jet has a more efficient bottom as the villain has a bit of a lip where the aluminum plate is(but helps increase pressure as it's like a ram water system), plus the villain has grate, delta jet didn't but has one bar now(might put in 2 more).

Comparision numbers (the amps are burst, not sure on constant draw(apparently I would need a data recorder to figure out constant) could be 40-60% of amp burst numbers)

feigao jet villain numbers (1640kv/rpm 50a motor)
14.8v again 29.1km/h (verified twice now at exactly same speed!) amp draw 45a = 666 watts or 3.04a per volt
11.1v 25.7km/h (good speed) amp draw 31.8a = 352 watts or 2.86a per volt
7.4v 15.6km/h (slow, really slow, not much fun) 14.85a draw = 109 watts or 2a per volt

Some chinese motor Purple motor 2300rpm/kv 50a
11.1v 64 amps approx 31.6km = 710 watts or 5.77a per volt
7.4v 39 amps 21.3km = 288 watts or 5.27a per volt

Need to think bout numbers now.... should I put feigao back in to delta jet? Or stick with current setup... esc currently supports 50a but have been thinking of doubling fets to allow for 100a.. on other hand feigao would run longer... mmmm... any votes?

Ivan.

edit point:

More I think about it, would be nicer if rpm was somewhere in between...[:@]

james94

is that what he used in the villain?

headhunter23

what do u mean? the feigao? btw was going to ask ya, would be cool to hook up, would like to get readings off ur boat sometime too, as it's in the higher rpm range that I believe would be better.

Ivan.

grael

[lightbulb]

He must have meant a [ SCALE ] speed of 80 km/hr

Now that could be right !

grael

Hi Ryan, (lipdaddy)

Ivan (headhunter) posted a link for kehrer, and I found a jet there that I would recomend as being a good size for your O.S. 40:
http://www.jet-drive.de/cms/componen...d,5/Itemid,17/

It's using a 34mm impellor, with a high pitch. Put a tuned pipe on your O.S., and you should be going pretty fast...

I'm using a 750mm hull, but in spins, it can get a bit of water inside from the sudden spin-stop. In my boat, as soon as I hit the gas, most of the water shoots to the back and out anyway, but If I was planning again from scratch, I would make the back even higher. I will take some pics to show you.

Graham.

headhunter23

scale? hmmm... maybe. what size boat would that be 3-4ft, original, maybe 30-40, say 1/10 scale... maybe I'm doing it wrong but wouldn't that mean 10x whatever?

grael

Your guess, 25-30km/hr,

According to website, "Gentry Eagle, Länge 1,70m, ca.80Km/h
Dieses Modell besticht durch seine Eleganz und Form sowie Speed, 80 Km/h
Art.Nr.: 17"

http://www.gentryeagle.com/page5.html give specs on the full sized one. It's 34.1 meters long, so if we simply do a linear back calculation:

1 kilometer / hr = 0.621371192 miles per hour
Real boat's speed: TOP SPEED AS A YACHT = 73 mph (63.47 knots)

73mph/0.621371192=117.48 km/hr

So, A scale speed based on a linar conversion would be 117.48 x (34.1/1.7) = 5.86 km/hr

In practice though, I would use the conventional hull maximum formula as a guide to scale speed calculations:
http://www.sailingusa.info/cal__hull_speed.htm
Key, is it's using the square root of the water line length.

So, I would recalculate like this:
scale boat speed = (scale boat length^.5/full size boat length^.5) x full size boat speed
=(1.7^.5/34.1^.5)*117.48
= 26.23 km/hr

Looks like a scale speed now eh !


note: programmers use the ^ symbol to denote to the power of, and to the power of 1/2, or 0.5 is the same as taking the square root of something.

Quicksilver

impressive math^^^

ok, so the PBR thing doesn't really pan out. I agree the 1/35 is too small, but go any bigger and it's big bucks. I want to built something fairly easy, but not done before, small(mini jet), and good for rapids, but possibly semi-scale. Those are some requirments. lol any ideas..the scale part isn't the most important, since it makes the build less easy.

grael

Hi Quick,
You could build a smaller version of the boat I designed and am running if you want, It's something I'm planning to do anyway.
If you were to do that though, you would be best to do balsa planking over the bulkheads, it's got some curves that sheet won't go over.

I need to do a better top for it, which will probably involve my setting up for vacuume forming, as it seems to be the best way to easily produce that sort of structure.

If you are looking for other ideas, you could do a cat, two speed controllers though...

There are also the larger IRBs (inflatable rubber boats) with fibre glass underside, and rubber tubes around the sides. That might be an interesting challenge, and you would have built in boyancy + shock absorbtion for the stones in rapids.

You could make the fabric up completely yourself if you wanted, start with some dynel or cotton, and roller brush latex solution on to it. If you did it over a sheet of glass, then one side would be glossy flat. You could mix a dye in with the latex, and have a fabric colour of your choice. After you have your general shape worked out, glue in a bicycle valve, and assemble...

I saw a jet hydrofoil somewhere recently, but the higher the rise of the water, the less efficient the jet would seem to be to me ? (and harder to prime)

Another possibility, would be to go purely for speed on a jet,

This site specifies some speeds:
http://www.performancejet.com/IMPELLERS.htm
Quote: "A/T jet drives have reached speeds of 163 MPH and have run 126MPH on the white water racing circuits. "

Now that's on a monohull I presume,
you won't get surface drive speeds, but you could get faster than a monohull jet if you went for a hydroplane design.

I'm not sure what pitch record breaking hydros use on their props, but they do have considerably wetted surface on their fins and rudder/s at speed, so a good jet design is not that far behind at all.

Let us know !

edit:
Just adding in, Latex is a relatively cheap building material too, cheaper than epoxy, polyester etc, anyway.

grael

Just another note about scale speeds,

We are intending to plane with most of our boats, but how it looks, compared to a full size boat has a lot to do with it's speed in the water, and how the water seperates from the hull.

I found this little gem on the net:
"The hull speed, by the way, can loosely be thought of as the speed at which the boat, in order to go faster, has to start ``climbing up'' over its bow wave, which takes a lot more power. (jfh) " (John F. Hughes 11/6/1997)

And somewhere else, I noticed that wave speed is a function calculated a bit like conventional hull maximum speed.

So, I think it is quite fair to use the formula I showed above, because how the boat looks in the water, bow wave, wake behind, attitude of the boat in the water, it's more to do with the behaviour of the water, and how fast the water can move out of the way, than a simple scaling of boat length.

In physics, they taught us about inverse square law, and I'm thinking that water doesn't just go away when you push it, it has a chance to move in three dimensions, and if you think of it as being like light intensity, if you double the distance, then you have only a 1.4 of the intensity over a similiar area. I have a feeling, that that will be why the relationship is non-linear, i.e., if you think about a boat's hull more in terms of area than in length, then identical design 1 meter and 2 meter boats, if the first boat has a footprint of 0.4 square meters, then the second will have a footprint of 1.6 square meters, not 1. The boat's weight is 4 x the smaller one's weight too.

Graham..

headhunter23

Hey quick do my idea of a cat with dual mini jets! Lemme know how it turns out! Decided I'm going to put the feigao back in, longer run times for a little less speed is doable, although I really like how she runs on 31km/hr, would love to hit that 40km barrier, maybe 6s might do it?

edit point, after writing that... am thinking why not test 5s and 6s to see what she does? Will need to soldier some deans together.

Ivan.

Quicksilver

a cat is tempting as well as a RIB. A RIB isn't exactly easy to build, but a cat has it's own problems. Something to look into. thanks

grael

Got my 2cc engine today, an ASP. Looks more like the old OS shape than the pic on the website, but it has the torque I'd hoped for from such a little engine.

Have to make my own mini jet now, part of the process is building my vacuum box. I tried an off the shelf sealed food container, but it imploded under vacuum, so yesterday I cut and glued the bottom siz sides of my new box. I'll be hooking it up to the intake of my compressor, and using a ply lid, foam rubber seals, and a thick acrylic window to peer in.

The whole idea, is that when I'm mixing silicone or urethane resins, I can mix properly, and then vacuum out the bubbles.

Else, I get voids, trapped bubbles, and weaknesses in the final piece/s. If I have those in my silocone mould, and then again in my urethane end piece, then I get twice the flaws...


Quick, an IRB, or as you call them, a RIB, I was thinking the hull should be easy, as it's less area, and has no undercuts, or could even be cut out of an old boat. If you slipped some glass fibre in with your latex/cloth sandwich, then you could prevent distortion when inflating. I think the latex would protect the glass, so long as there was also some cloth both sides. Latex looses thickness as it dries, but bonds to itself readily. It's also used in a runny solution in the bathroom tile/grouting business, as a binder and plasticiser for the harder compounds. And, UHU, cyano glue, several would bind it well to your hull material.

Quicksilver

you do have a point, I've got that yellow jet boat laying around, that I'd gladly hack up and I've got a sewing machine. have to think about this [8D]

headhunter23

Nugh new numbers por jet villain.

18.5v does 32.4km/h @ 53.75a total 994 watts
22.2v does 32.3,32.8km/h on 63.3a (two readings because I couldn't believe I only did .4km faster than 18.5v, second reading was slower) total 1405 watts

I guess I hit max speed with grate and villain. Have to change the aluminum plate to remove waddle that she's been doing ever since I redid the back. I'm sure without grate it would be faster... but still... not much more speed for more power.

Edit point, made little chart, seems optimum speed in terms of voltage is around 28-29k/hr....

DRAGONFIRE81M

sweet love the info there headhunter thats on the kmb drive right ?

headhunter23

yep kehrer, hey smokerete u got vids o ur boat running?

grael

Ivan,
I have a theory,
plastic impeller, right ?

Perhaps it's flexing too much, and loosing effectiveness.

Try testing the static thrust with some fishing scales, and then, using the same thrust force, push on the faces of the impeller blades on the trailing edge, just to see how much deflection you get. If they lift off the stator too much, your slippage will increase substantially, which could account for your poor speed increase.

Another thing to look at, if you are going quite fast, is parasitic hull vibrations. If you take a long skinny piece of paper and stick it in the end of a vacuum cleaner, you will hear a noise, and the paper will undulate fast, with sine waves traveling down the paper in the direction the air is moving. Same thing happens with water on a surface, and wind on the surface of water (waves), and water in a river, carving undulating banks.

So, feel the hull, consider whether it might be vibrating at speed. If so, then glue some carbon fibre down the flimsiest sections, parallel with the hull.

If it's parasitic vibration drag, then it will increase substantially for even small hull speed increases. It might even account for the boat duck walking, if it drags on one side and then the other.

Graham

pinky_d_brain

Hi guys I have bean checking this thread out for a wile vary cool stuff [8D] you guys need your own section on here or at least make this a sticky.
I have no experience with jet drives but I have bean thinking, the surface of the water is where you get all your thrust from correct. If you submerge a jet drive the water is just kind of absorbed by the larger body of water around it, like if you submerge a shower head in a bath tub there is les force then if it simply sprayed on the water. So if you increased the area of the spray on the water would that not provide you with more thrust, by increasing the surface area that the water would be sprayed across to create traction on the water? Instead of a single point of contact with the water you could have multiple or simply a wider spray increasing your traction making the jet drive more efficient. I was thinking instead of a round nosel make it an oval shape to spread it out a bit. Anyway just kind of thinking out loud don’t even no if it makes sense to anyone

headhunter23

ur right about the wagging affecting the speed, flex of the prop is possible. Grate is certainly affecting stuff though. Still not really an increase in speed at all, I know the hull is good for 40-50mph, so shouldn't be that. Was supposed to chop it and fix the alumiunum tab on the back today but not going to get to it.

Maybe tomorrow... will see. As for spreading the exhaust of the jet, not 100% sure but I think the focused jet allows for a faster speed, better turning and outta the hole. Ask quick or dragon bout that stuff as they've both screwed around with nozzles.

Ivan.

grael

Did you take a video ?

A lot of larger hulls these days are fitted with stringers to mount the engines on, but few would do that with electric. The stringers mke the hull more rigid where it counts.

I put 200grams of expanding foam into my boat, mostly the front, not so much for bouyancy, but to keep the bow from lifting so easily.

I still need to get some video of mine, ran it again today, and took 2 new Fox glow plugs down with me, as the fox plug I had in it was cooked (the pins go loose if the engine gets hot. So, problems starting, replaced glow plug battery, stil no go. Checked plug, it was getting hot, but from resistance of iron in it. The coil wasn't heating at all. So... dud plug. tried my other new plug (both fresh out of packet), no go with that one either. Took it out, checked, no heating on coil... coil fitted crookedly at factory. Tried to straighten, by the nichrome to plug weld broke straight away.

Went back home, grabbed the other new plug from my new 2cc ASP, and a spare O.S. plug that I wasn't sure idled properly.

Got one good run out of the engine, but then it wouldn't start again. Checked the almost new Fox plug- dead.

Fitted the O.S. in, and ran three tanks through the engine, crowd gathering, as usual... Every time I take it out now, people ask me how much they cost to buy.

It's wickedly fast, I'm guessing more than 55km/hr. Any time I slow it, put it into a turn and hit the throttle again, it leaps out of the water. I can't turn it full speed, other than very gradually, because it will spin out. I've changed the shape of the steering nozzle, to provide some downforce when turning, and it's helped. Probably, I need a couple of tiny turn fins about 1/5th forward of the stern, and angled to grab in when the sides dip down.

About pinky's comment, I know what he means. If you are holding a fire hose and tighten the nozzle to make the jet change to spray, it's a lot harder to hold. Difference is, we are matching an engine/motor to our pumps, and if we restrict the nozzle to increase the exit speed, then we overload the engine/motor. Also, we are using axial flow or mixed flow pumps, not positive displacement. The more we restrict flow, the more slippage and inefficiency we have. I find less restriction, and a high pitch impeller work a treat for me.

headhunter23

leme get a good wad and spit on stringers... for the extra weight it would be better spent spray foamin the entire boat as it would be just as stiff but unsinkble. joking aside though i think i've just topped out the motor with the setup is all. I would like to see a 50km jet drive system though.... post vids as I didn't.

Ivan.

Quicksilver

Well, I dont think we want to widen the spray area. You lose thrust, unless you severely compensate with more power i.e. greater jet velocity. Our jets if used on a planing hull, aren't going to be under water at all. With say a 1/10 scale boat at speed, were only using something like the top 1/8" of the water column. At 25+mph it doesn't take much water to keep that pump primed and still have the motor using much less than it's full potential. As I've said many times before, yes we do need less slippage and tighter pumps. I agree that tightening the bowl isn't going to necessarily make the boat go faster due many to slippage and I'm sure blade deflection isn't helping here as well.

With say the 28 or 29mm pumps, more pitch could give you something, but you'll loose some acceleration and I'm thinking it must be easier to lose prime.

What I would do next and I guess this is aimed at headhunter, try the graupner 40mm. It's a 29mm drive with a bigger intake, same duct and then a 25% larger impeller, which I would think would address the bowl restriction idea. Should provide more velocity, since basically the only thing different is the impeller size, so more flow. In fact I wouldnt be surprised if you could fit the 40mm impeller and bowl on the back of the 29mm or graupner or with some cutting, onto the 28mm KMB duct.